Gas turbine electric power plants are utilized in so-called base load, mid-range load and peak load power system applications. Combined cycle plants are normally used for the base or mid-range applications while the power plant which utilizes a single gas turbine as the generator drive is highly useful for peak load applications because of its relatively low cost.
In the operation of gas turbines, particularly in electric power plants, various kinds of control systems have been employed from relay-pneumatic type systems, to analog type electronic controls, to digital controls, and more recently to computer based software controls. U.S. Pat. No. 4,308,463--Giras et al., assigned to the assignee of the present invention and incorporated herein by reference, lists several of such prior systems. That patent also discloses a digital computer based control system for use with gas turbine electric power plants. It can be said that the control system described in U.S. Pat. No. 4,308,463 is a predecessor to the system described in the present invention. It will be noted that the Giras et al. patent is one of a family of patents all of which are cross referenced therein.
Subsequent to the Giras et al. patent, other control systems have been introduced by Westinghouse Electric Corporation of Pittsburgh, Penna. under the designations POWERLOGIC and POWERLOGIC II. Similar to the Giras et al. patent these control systems are used to control gas turbine electric power plants. However, such control systems are primarily micro-processor based computer systems, i.e. the control systems are implemented in software, whereas prior control systems were implemented in electrical and electronic hardware.
The operating philosophy behind the POWERLOGIC and POWERLOGIC II control system is that it shall be possible for the operator to bring the turbine generator from a so-called ready-start condition to full power by depressing a single button. All modes of turbine-generator operation are to be controlled including control of fuel flow at the point of closure of the generator breaker up to the attainment of base load.
The present invention constitutes an improvement on the POWERLOGIC II system. In the above listed prior combustion turbine-generator control systems, fuel flow from the point of closure of the generator breaker to base load was achieved by increasing a turbine speed setpoint reference at a preset rate. Such a control scheme would result in a positive error on a proportional integral differential (PID) controller causing an increase in turbine power and thus an increase in generator load. Generator load would increase until a minimum generator load was detected, i.e. until a kilowatt limit was reached. Unfortunately, due to total system response time the fuel control tends to oveshoot the minimum load setpoint. Consequently, maintaining generator load at a preset minimum load required operator intervention. Therefore, a need still exists for a turbine-generator control scheme which will automatically maintain generator load at a preset minimum without operator intervention.
Although, the operation of a gas turbine electric power plant and the POWERLOGIC II control system are described generally herein, it should be noted that the invention is particularly concerned with the control of fuel in the gas turbine and specifically is an improvement to the control of fuel flow at the point of closure of the generator breaker.